294 THE DEPTHS OF THE SEA. [CHAP. VII. 
end is attached to the base of a short cylinder. 
Any variation of temperature causes the coil to 
wind or unwind, and its motic» acts to rotate the 
axial stem. This motion is magnified by multiply- 
ing wheels, and is registered upon the dial of the 
instrument by an index which pushes before it a 
registering hand, moving with sufficient friction 
merely to retain its place when thrust forward by 
the index hand of the thermometer. The instru- 
ment is graduated by trial. The brass and silver 
portions are thickly gilt by the electrotype process 
to prevent the action of sea-water upon them. The 
box which covers the coil and indicatory part of the 
thermometer is merely to protect it from accidental 
injury, and is open so as to permit the free passage 
of the sea-water. This instrument appears tosanswer 
tolerably well for moderate depths, its error up to 
600 fathoms not greatly exceeding 0°5C.; at 1,500 
fathoms, however, the error rises to 5°C., quite as 
great as that of the unprotected Six’s thermometers, 
and the error is not so constant. It is evident 
that under great pressure little confidence can be 
placed upon instruments which give their indica- 
tions through metal machinery. 
Before H.M.S. ‘ Porcupine’ started on her summer 
cruise in 1869, a valuable series of experiments were 
made upon the effect of pressure on various register- 
ing thermometers at Woolwich, under the superin- 
tendence of the Hydrographer and of the Deep-Sea 
Committee of the Royal Society. The object was to 
subject all the forms of deep-sea thermometers in use 
to pressures in a hydraulic press, equivalent to the 
pressures which they would encounter at different 
